The hardening of workpieces made of steel, in particular made of a manganese-boron steel, is a sufficiently well-known method for adapting material properties. To this end, the workpiece is heated, depending on the material, to its austenitizing temperature, for example to about 930° C. in the case of a manganese-boron steel, and then quickly cooled or quenched. In order to produce, for example, bodywork components, the workpieces to be hardened are inserted into a hardening tool for cooling. Here, the workpiece undergoes at most a very small change in shape by calibration or pressing.
The fundamentals for the shape of the hardened workpiece are specified, in the case of indirect hot forming, in particular initially by cold forming. If a hollow profile is intended to be subjected to the hardening process, then the hardening tools generally merely ensure cooling of the workpiece via the outer face of the hollow profile.
A method in which a coolant in a hardening tool is made to pass through an interior space of the hollow profile and in the process to cool the hollow profile from the inside is known from the document EP 1 755 801 B1. However, the method is restricted to the hardening of hollow profiles which have also been shaped in the hardening tool.
Furthermore, workpieces made of a manganese-boron steel have a tendency for scale formation. Therefore, these steels are provided with oxidation-preventing layers such as AlSi, AlMg, zinc or other layers. These layers meet the desired requirements, but result in undesired side-effects, such as longer furnace residence times for inward diffusion, mixing with a material during a welding operation and/or additional costs as a result of the application and local removal of the layers on the manganese-boron steel.
Therefore, the prior art proposes carrying out heating operations on only uncoated workpieces and then removing the scale that has developed by way of a suitable method, for example by sand blasting. However, it has been found that subsequent removal of the scale is associated with higher additional costs, in particular for hollow profiles, the interior spaces of which, having a comparatively small cross section and a long length, can be reached only with difficulty. In addition, the scarcely avoidable soiling with scale residues results in increased cleaning outlay and increased tool wear.